This invention relates to a catalytic process for hydrotreating heavy hydrocarbon streams containing asphaltenic material, metals and sulfur compounds. More particularly, this invention relates to hydroprocessing using multiple-stage catalytic treatment with catalyst having improved effectiveness of activity maintenance in the desulfurization of metal-containing hydrocarbon streams.
As refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for processes to treat the fractions containing increasingly higher levels of metals, asphaltenes, and sulfur.
It is widely known that various organometallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbon streams derived from tar sands, and hydrocarbon streams derived from coals. The most common metals found in such hydrocarbon streams are nickel, vanadium, and iron. Such metals are very harmful to various petroleum refining operations, such as hydrocracking, hydrodesulfurization, and catalytic cracking. The metals and asphaltenes cause interstitial plugging of the catalyst bed and reduced catalyst life. The various metal deposits on a catalyst tend to poison or deactivate the catalyst. Moreover, the asphaltenes tend to reduce the susceptibility of the hydrocarbons to desulfurization. If a catalyst, such as a desulfurization catalyst or a fluidized cracking catalyst, is exposed to a hydrocarbon fraction that contains metals and asphaltenes, the catalyst will become deactivated rapidly and will be subject to premature replacement.
Although processes for the hydrotreating of heavy hydrocarbon streams, including but not limited to heavy crudes, reduced crudes, and petroleum hydrocarbon residua, are known, the use of fixed-bed catalytic processes to convert such feedstocks without appreciable asphaltene precipitation and reactor plugging and with effective removal of metals and other contaminants, such as sulfur compounds and nitrogen compounds, are not common because the catalysts employed have not generally been capable of maintaining activity and performance.
While multiple-stage catalytic processes for first hydrodemetallation followed by hydrodesulfurization treatment of heavy hydrocarbon streams having a high metals content are known, catalyst deactivation continues to be generally prohibitive to commercial application; particular difficulty has been experienced in the deactivation of the desulfurization catalyst primarily because conventional desulfurization catalysts containing Group VIII metal, particularly cobalt, have been relied upon, resulting in insufficient catalyst life, where metals in the hydrocarbon feedstock have caused desulfurization activity of the catalysts to deteriorate. Examples of multiple-stage catalytic processes for hydrotreatment of heavy hydrocarbon streams containing metals are disclosed in U.S. Pat. Nos. 3,180,820 (Gleim et al., 1965); 3,730,879 (Christman, 1973); 3,977,961 (Hamner, 1976); 3,985,684 (Arey et al., 1977); 4,016,067 (Fischer, 1977); 4,054,508 (Milstein, 1977); 4,051,021 (Hamner, 1977) and 4,073,718 (Hamner, 1978).
The catalysts disclosed in these references contain hydrogenating component comprising one or more metals from Group VIB and/or Group VIII on high surface area support such as alumina, and such combinations of metals as cobalt and molybdenum, nickel and molybdenum, nickel and tungsten, and cobalt, nickel, and molybdenum have been found useful. Generally, cobalt and molybdenum have been preferred metals in the catalysts disclosed for hydrotreatment of heavy hydrocarbon streams, both in first-stage catalytic treatment to primarily remove the bulk of the metal contaminants, and in second-stage catalytic treatment primarily for desulfurization. None of the references disclose actual examples of processes employing catalyst containing only Group VIB metal in the second-stage catalyst, and none of the references has suggested that desulfurization activity can be maintained and the life of the desulfurization catalyst can be improved if the catalyst contains only Group VIB metal.
Hopkins et al. in co-pending United States patent application Ser. No. 811,835, filed June 30, 1977, now U.S. Pat. No. 4,119,531 which is incorporated herein by reference, disclose a process for hydrodemetallation of hydrocarbon streams containing asphaltenes and a substantial amount of metals, which comprises contacting the hydrocarbon stream with a catalyst consisting essentially of a small amount of a single hydrogenation metal from Group VIB or Group VIII, deposed on a large pore alumina; suitable examples of the hydrogenation metal are nickel or molybdenum. The catalyst is characterized by a surface area of at least 120 m.sup.2 /gm; a pore volume of at least 0.7 cc/gm, and an average pore diameter of at least 125 A units.
Hopkins et al. suggest that while hydrodemetallation of heavy hydrocarbon streams is improved by employing catalyst consisting essentially of a single Group VIB or Group VIII hydrogenation metal, the substantially demetallated effluent will not normally be sufficiently desulfurized for further refining processes; consequently, there is great need for durable, effective desulfurization catalyst for use in processing the substantially demetallated stream.
The general object of this invention is to provide an improved process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing metals.
Another object of this invention is to improve the maintenance of activity of catalyst employed in the hydrodesulfurization stage of hydrodemetallation-hydrodesulfurization treatment of metal-containing heavy hydrocarbon streams.
We have found that the objectives of this invention can be attained by a sequential, two-stage hydrotreatment of metal-containing heavy hydrocarbon feedstock wherein a demetallation catalyst employed in the first stage provides demetallated effluent which is contacted in the second stage with the desulfurization catalyst containing at least one original Group VIB metal deposed on an alumina support. We have found for example that as little as 2.2 wt.% cobalt oxide causes rapid deactivation of the second-stage catalyst for sulfur removal; as a result of omitting from the second-stage desulfurization catalyst the conventional Group VIII component, the deactivating effect of the Group VIII metal, particularly cobalt, is eliminated and the process of this invention will achieve significantly improved combination of hydrodemetallation and hydrodesulfurization of the metal-containing heavy hydrocarbon streams, with greatly extended life of the second-stage, hydrodesulfurization catalyst, even under severe conditions of operation. The combined effect of substantially demetallating the feed with effective catalyst in the first stage, together with the elimination of the deactivating influence of Group VIII metal in the second-stage catalyst accomplishes the particularly effective preservation of the desulfurization activity of the second-stage catalyst and the improved length of the on-stream service permitted by the process of this invention.
Briefly, this invention comprises a two-stage process for hydrometallation and hydrodesulfurization of a hydrocarbon feedstock containing asphaltenes and a substantial amount of metals. The first stage of this process comprises contacting the feedstock in a first reaction zone with hydrogen and a demetallation catalyst typically comprising hydrogenation metal selected from Group VIB and/or Group VIII deposed on a large-pore, high-surface area inorganic oxide support, suitably alumina, silica, magnesia, zirconia and similar materials; the first-stage catalyst has a surface area of about 120 m.sup.2 /gm to about 400 m.sup.2 /gm, an average pore diameter within the range of about 125 A to about 350 A, and a pore volume within the range of about 0.7 cc/gm to about 1.5 cc/gm. The second stage of this process comprises contacting the effluent from the first reaction zone with a catalyst consisting essentially of at least one active original hydrogenation metal selected from Group VIB deposed on a smaller-pore, catalytically active support comprising alumina, and said metal being in at least one form selected from the group consisting of the elemental form, the oxide, and the sulfide. The catalyst has a surface area within the range of about 150 m.sup.2 /gm to about 300 m.sup.2 /gm, an average pore diameter within the range of about 90 A to about 160 A, and the catalyst has a pore volume within the range of about 0.4 cc/gm to about 0.9 cc/gm.
The preferable pore volume distribution for the second-stage catalyst is summarized as follows:
______________________________________ Pore Diameters, A % of Pore Volume ______________________________________ 50-80 &lt;40 80-100 15-65 100-130 10-50 130+ &lt;15 ______________________________________
We have found that it is surprisingly important to the desulfurization performance of the second-stage catalyst of this invention that maximum surface area exists as shown in FIG. 5, in the catalyst pores having diameters in the range of about 80-130 A; preferably the second-stage catalyst has about 90 m.sup.2 /gm to about 180 m.sup.2 /gm surface area in 80-130 A pores and more preferably such pores contain about 115-180 m.sup.2 /gm.
The term "active original hydrogenation metal" is used herein to refer to only the hydrogenation metal that is incorporated into the catalyst during its preparation and does not include any metal that is deposited upon the catalyst during the use of the catalyst in any process. Molybdenum, which is generally superior to chromium and tungsten in demetallation and desulfurization activity, is a preferred Group VIB metal component in both the first-stage catalyst and the second-stage catalyst. While generally Group VIB metal provides superior demetallation activity in comparison to Group VIII metal, nickel is a preferred Group VIII metal component in the first-stage catalyst.
The support for both first-stage catalyst and second-stage catalyst of this invention is preferably alumina; however, the support can comprise silica, phosphate, or other porous refractory inorganic oxide, preferably in an amount less than about 5 wt.% of the support.
In both stages or reaction zones, catalysts may be employed in the form of a fixed-bed or an ebullated bed of particles. In the case of a fixed-bed, the particulate material catalyst should have a particle size of at least 1/32 inch effective diameter.
Broadly, the present invention is directed to a process for the hydrotreating of heavy hydrocarbon feedstocks. Such feedstocks will contain asphaltenes, metals, nitrogen compounds, and sulfur compounds. It is to be understood that the feedstocks that are to be treated by the process of the present invention will contain from a small amount of nickel and vanadium, e.g., about 40 ppm, up to more than 1,000 ppm, of the combined total amount of nickel and vanadium and up to about 25 wt.% asphaltenes. This process is particularly useful in treating feedstock with a substantial amount of metals containing 150 ppm or more of nickel and vanadium and a sulfur content in the range of about 1 wt.% to about 10 wt.%. Typical feedstocks that can be treated satisfactorily by the process of the present invention will also contain a substantial amount of components that boil appreciably above 1,000.degree. F. Examples of typical feedstocks are crude oils, topped crude oils, petroleum hydrocarbon residua, both atmospheric and vacuum residua, oils obtained from tar sands and residua derived from tar sand oil, and hydrocarbon streams derived from coal. Such hydrocarbon streams contain organometallic contaminants which create deleterious effects in various refining processes that employ catalysts in the conversion of the particular hydrocarbon stream being treated. The metallic contaminants that are found in such feedstocks include, but are not limited to, iron, vanadium, and nickel.
Nickel is present in the form of soluble organometallic compounds in most crude oils and residuum fractions. The presence of nickel porphyrin complexes and other nickel organometallic complexes causes severe difficulties in the refining and utilization of heavy hydrocarbon fractions, even if the concentration of such complexes is relatively small. It is known that a cracking catalyst deteriorates rapidly and its selectivity changes when in the presence of an appreciable quantity of the organometallic nickel compounds. An appreciable quantity of such organometallic nickel compounds in feedstocks that are being hydrotreated or hydrocracked harmfully affects such processes. The catalyst becomes deactivated and plugging or increasing of the pressure drop in a fixed-bed reactor results from the deposition of nickel compounds in the interstices between catalyst particles.
Iron-containing compounds and vanadium-containing compounds are present in practically all crude oils that are associated with the high Conradson carbon asphaltic and/or asphaltenic portion of the crude. Of course, such metals are concentrated in the residual bottoms, when a crude is topped to remove those fractions that boil below about 450.degree. F. to 600.degree. F. If such residuum is treated by additional processes, the presence of such metals adversely affects the catalyst in such processes. It should be pointed out that nickel-containing compounds deleteriously affect cracking catalysts to a greater extent than do iron-containing compounds. If an oil containing such metals is used as a fuel, the metals will cause poor fuel oil performance in industrial furnaces, since they corrode the metal surfaces of the furnaces.
While metallic contaminants, such as vanadium, nickel, and iron, are often present in various hydrocarbon streams, other metals are also present in a particular hydrocarbon stream. Such metals exist as the oxides or sulfides of the particular metal, or they are present as a soluble salt of the particular metal, or they are present as high molecular weight organometallic compounds, including metal naphthenates and metal porphyrins, and derivatives thereof.
Sequentially hydrotreating heavy hydrocarbon feedstock with the first-stage catalyst of this invention followed by the second-stage catalyst of this invention will allow hydrodemetallation and hydrodesulfurization with greatly extended catalyst life even under severe conditions.